Abstract

The interferometric monitor for greenhouse gases (IMG) was the
precursor of the high-resolution Fourier-transform infrared radiometer
(FTIR) onboard a satellite for observation of the Earth. The
IMG endured the stress of a rocket launch, demonstrating that the
high-resolution, high-throughput spectrometer is indeed feasible for
use onboard a satellite. The IMG adopted a newly developed
lubricant-free magnetic suspension mechanism and a dynamic alignment
system for the moving mirror with a maximum traveling distance of 10
cm. We present the instrumentation of the IMG, characteristics of
the movable mirror drive system, and the evaluation results of sensor
specifications during space operation.

Figures (5)

Schematic of the IMG optical system. The optical
aperture of the interferometer was 10 cm in diameter. The
interferometer was composed of flat mirrors and a large KBr beam
splitter. The moving mirror was suspended magnetically, and the
alignment was controlled dynamically with a He–Ne laser interferogram
equipped with four Si detectors. The optical system did not have an
intermediate cold field stop.

Outline of the detector component. The vacuum vessel
contained the cold field stop and the cold focal plane upon which were
placed three detectors and three bandpass filters. The cold finger
of the cryogenic cooler displacer was attached to the back of the
vessel.

Moving mirror alignment and control system
diagram. The control system has double control loops. In the
basic loop, the moving mirror armature was suspended and the
parallelism was sustained with a predefined alignment bias. The
second loop supported the dynamic alignment throughout the observation
by use of the interference signal of the He–Ne laser.

IMG-observed and calculated atmospheric spectral
radiances. The solid curve represents the IMG-observed spectrum and
the dashed curve represents the theoretically calculated spectrum with
the objective analysis temperature and humidity profile interpolated in
space and time to the IMG observations. The model8
that we used for this calculation is fascode with the
spectroscopic HITRAN 92 database.9 We can retrieve
atmospheric constituent profiles10 by minimizing the
difference between the measured and calculated spectra.

a The longer wavelength region of the
spectral range was expanded after the data were evaluated. The Band
3 has a redefined range of 667–2000 cm-1; the delivered
data range is now 600–2000 cm-1.b In kilobits per second.c Except for the remote interface unit.

a The longer wavelength region of the
spectral range was expanded after the data were evaluated. The Band
3 has a redefined range of 667–2000 cm-1; the delivered
data range is now 600–2000 cm-1.b In kilobits per second.c Except for the remote interface unit.